Compact linear accelerator with accelerating waveguide

ABSTRACT

A linear accelerator head for use in a medical radiation therapy system can include a housing, an electron generator configured to emit electrons along a beam path, and a microwave generation assembly. The linear accelerator head may include a waveguide that is configured to contain a standing or travelling microwave. The waveguide can include a plurality of cells that are disposed adjacent one another, wherein each of the plurality of cells may define an aperture configured to receive electrons therethrough. The linear accelerator head can further include a converter and a primary collimator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/276,323, filed Feb. 14, 2019, entitled “COMPACT LINEAR ACCELERATORWITH ACCELERATING WAVEGUIDE,” which is a continuation of U.S. patentapplication Ser. No. 15/933,257, filed Mar. 22, 2018, entitled “COMPACTLINEAR ACCELERATOR WITH ACCELERATING WAVEGUIDE,” which claims thebenefit of U.S. Provisional Application Ser. No. 62/476,630, filed Mar.24, 2017, entitled “COMPACT LINEAR ACCELERATOR WITH ACCELERATINGWAVEGUIDE,” all of which are incorporated by reference herein in theirentirety and for all purposes.

BACKGROUND Field

The present disclosure relates to radiation therapy, in particular tobeam generation and beam hardware.

Description of the Related Art

Modern radiation therapy techniques tend to rely on bulky machinery witha limited scope of volumetric angles at which therapy can beadministered.

Systems and methods disclosed herein address various challenges relatedto photon therapy and photon generation.

SUMMARY

Described herein include embodiments related to linear accelerators(“linacs”) and related components. A linac is a device commonly used forexternal beam radiation treatments for patients with diseases, such ascancer. Some linear accelerators may be used to treat a broad range ofangles (e.g., solid angles) around a patient, which may include all bodyparts and/or organs of the patient. The linac can be configured todeliver high-energy (e.g., 6 MV) radiation (e.g., x-rays) to the regionof the patient's body, such as tumors.

A linac head may include a compact accelerating waveguide. The systemmay also include other hardware components (e.g., magnetron, cooling,waveguide isolators, waveguide plenums, sensors). The linac head may beconfigured to move in a broad range of solid angles (e.g., anear-complete spherical outline) around a standard patient bed (e.g.,couch). As will become clear from the following disclosure, producing aneffective compact waveguide and linac head can present a variety oftechnical challenges, which may be solved by many of the novel designfeatures disclosed herein.

The accelerating waveguide can be housed in a small volume. For example,the isolator may be disposed in a parallel configuration with theaccelerating waveguide. As a further example, the magnetron may beperpendicular to the isolator and/or the accelerating waveguide. Thelinac may be fit into a compact housing. The compact configuration ofthe linac may increase clinical efficiency by allowing the device (withthe assistance of a mechanical (e.g., robotic) arm) to be maneuveredwith six or more degrees of freedom inside of a treatment room. Theradiation device may be compact enough to treat a patient across a largerange of angles in order to maximize the dose to the target treatmentvolume (e.g. tumor) while minimizing the dose to healthy issues bydynamically positioning the device relative to the patient. The housingof the various components of the radiation device may allow themechanical arm to provide treatment to areas that were previouslyunreachable and/or tailor a dose profile to conform as much as possibleto the tumor volume. The software that develops the treatment plan thatspecifies the angles and positions to deliver radiation from, as well asthe shape of the radiation as manipulated by dynamic Multileafcollimators, can be programmed to minimize impact to healthy and/orradiation sensitive tissues. Increasing dose conformity can allowboosting the dose to the treatment volume, allowing higher probabilityof tumor control

Some embodiments of linac systems can minimize the size of anaccelerating waveguide that is capable of achieving the energy and doseoutput required for radiation therapy. This may be accomplished by usingmultiple waveguide cells that are organized linearly. In order to reducethe size, the system may include one or more side cells for coupling RFpower between the adjacent accelerating cells. In some designs, one ormore cells have a side tuner configured to allow tuning of each of theone or more cells so that the waveguide can provide increased RF powerand hence increased electron acceleration. In some embodiments, thistuning may be available only once shortly after manufacture of thevarious components of the system (e.g., parts of the waveguide).

Since the power is high in a smaller space, a novel cooling system maybe integrated into the waveguide.

The power of the electrons and/or microwaves that the linac generatescan be high. In some embodiments, the apertures within the acceleratingwaveguide are small. Because the waveguide is relatively short and/orbecause of the size of the waveguide apertures, novel vacuum ports canbe included that can provide relatively high vacuum. Two vacuum ports(e.g., one on each end of the waveguide) may be included to improvevacuum pumping.

The design of the various components of a linac head may be arranged ina way that reduces the overall size of the linac head. Accordingly, amore compact standing waveguide can be installed in the linac. Thecompact size and/or light weight of the linac can allow a mechanical armto move the linac head around a patient in a treatment bed and/or withina treatment room through a range of angles comprising up to 360-degreesabout a first axis defined by the intersection of the x- and y-planes aswell as 360-degrees about an orthogonal axis. Previous linac machineshave unusable and/or unreachable angles of therapy delivery. Moreover,the waveguide cells can be manufactured to optimize the power that isavailable from a combination of microwave generator (e.g., magnetron)and/or electron generator (e.g., electron gun) using specific sizes,shapes, and/or tunings.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present disclosure will now be described, byway of example only, with reference to the accompanying drawings. Fromfigure to figure, the same or similar reference numerals are used todesignate similar components of an illustrated embodiment.

FIG. 1 is a schematic diagram illustrating components of a linac head inaccordance with an exemplary embodiment.

FIG. 2 is a cutaway perspective drawing of the linac head of FIG. 1.

FIG. 3 illustrates an energy level range of electrons passing through awaveguide aperture.

FIG. 4 illustrates an example coupling of components of an example linachead.

FIG. 5A is a cutaway perspective drawing of an electron gun vacuumflange.

FIG. 5B is a cutaway perspective view of the electron gun vacuum flangeof FIG. 5A installed within a linac head.

FIG. 6 illustrates an example standing waveguide.

FIG. 7 is a cutaway perspective view of an example standing waveguide.

FIG. 8 illustrates an example internal geometry of a standing waveguide.

FIG. 9 illustrates the effects of heating on the geometry of a standingwaveguide.

FIG. 10 is a cross-sectional view illustrating the electric fielddistribution in an example group of bunching cells and an acceleratingcell.

FIG. 11 is a cross-sectional view illustrating the effect on electricfield distribution of tuning a waveguide by altering the shape of acell.

FIGS. 12A-12C illustrate an example standing waveguide with cell tuningcomponents.

FIGS. 13A and 13B illustrate example geometries of waveguide cellsincluding side cells.

FIG. 14 illustrates tuning structures of an example linac waveguide.

FIG. 15A illustrates an example cooling system for a linac waveguide.

FIG. 15B depicts a temperature distribution of a linac waveguide cooledwith the cooling system of FIG. 15A.

FIG. 16A depicts an alternative cooling system configuration for a linacwaveguide.

FIG. 16B depicts a temperature distribution of a linac waveguide cooledwith the cooling system of FIG. 16A.

FIG. 17 is a perspective view of an example vacuum flange for a linacwaveguide.

FIG. 18 is a cutaway perspective view of a converter and a convertervacuum flange installed in a linac waveguide.

FIG. 19 is a perspective view of an example primary collimator for alinac waveguide.

FIG. 20 schematically depicts an example treatment system configurationincluding the linac systems described herein.

DETAILED DESCRIPTION

Various systems and individual components, as well as associatedmethods, are disclosed herein. In some embodiments, a system is composedof four main modules: a linac head, which may contain a radio frequency(RF) power source and a linac waveguide (e.g., standing waveguide(SWG)); an electronics rack, which may include a gun deck to power anelectron gun, a modulator to power a microwave generator (e.g.,magnetron), and other elements; a temperature control unit (TCU), whichmay distribute coolant (e.g., water) in a closed-loop for powerdissipation; and a control console, which may be integrated into theelectronics rack and/or configured as a standalone unit to allow forplacement in a remote location.

The details of the design of the various subsystems of some embodimentsare included in the following sections.

Linac Head

FIGS. 1 and 2 depict example compact linac heads and components thereof.FIG. 1 schematically illustrates the various components of the linachead 100. FIG. 2 is a cutaway perspective drawing illustrating exampleelements as installed in the linac head 100 and/or system. Some designsuse a linear accelerating structure (e.g., accelerating waveguide 110)using microwaves at one or more ranges of frequency (e.g., X band, Sband), which may be between about 7.0 and 11.2 GHz, such as, forexample, at approximately 9300 MHz, to accelerate electrons at one ormore ranges of kinetic energy (e.g., between about 2 MeV and 8 MeV).Other designs can be used to accelerate electrons having higher or lowerenergies. In some embodiments, a photon (e.g., x-ray) beam with a doserate between about 300 cGy/min and 1,800 cGy/min (e.g., approximately1,000 cGy/min) can be generated at about 1 m from the X-ray converter.

Referring jointly to FIGS. 1 and 2, an example embodiment of a linearaccelerator head 100 for use in a radiation therapy system can include ahousing 101, an electron generator 105, a waveguide 110 configured tocontain a standing or travelling microwave, the waveguide comprising aplurality of cells disposed adjacent to one another, wherein each of theplurality of cells comprises an aperture configured to allow electronsto pass therethrough, a converter 120 disposed along a beam path of theelectrons and configured to convert electrons to photons, and a primarycollimator 125 configured to shape the resulting X-ray beam.

The linac head 100 further includes a vacuum pump 115 for removingparticles, gases, and the like, from the waveguide 110 through anelectron gun vacuum flange 117 and a converter vacuum flange 119. Amicrowave generator 130 produces microwave-range radiation, which can becoupled into the waveguide 110 through a RF window 140 and coupler 145.An isolator 135 transmits the RF power received from the microwavegenerator 130 to the RF window 140 while preventing RF power from beingtransmitted in the opposite direction, back to the microwave generator.As shown in FIG. 2, the compact linac head may be mounted on amechanical arm 150. The various components described above and depictedin FIGS. 1 and 2 will be described in greater detail below.

Microwave Generator

As shown in FIG. 2, a microwave generator produces a microwave signalthat may be coupled into the waveguide at a coupler 145. A microwavegeneration assembly may include a microwave generator (e.g., magnetronor the like). In some embodiments, the microwave generator can be anoscillator that converts DC pulses from a modulator into RF power. Asdescribed above, the microwave generator may produce a power greaterthan 1.5 MW. The microwave generator may include a tuning mechanism toadjust an output frequency. In some designs, the tuning mechanism mayallow the output frequency to be tuned from between about 9.27 GHz to9.33 GHz. The microwave generator may include a water-cooled anodeand/or focusing magnet. A filament of the microwave generator may beconfigured to supply over 2,000 hours of operation. The microwavegenerator may further include an ion pump. In some embodiments, the ionpump can produce about 0.4 l/s.

Isolator

An isolator may be included in the microwave generation assembly. Theisolator can be configured to prevent microwaves from traveling backinto the microwave generator. The isolator may have two or more loadsattached to it (e.g., it may have 4 ports on it). For example, it mayinclude a main pass and/or two auxiliary ports.

RF Window

The microwave generation assembly may include an RF window. An RF windowmay be configured to separate gas inside the microwave generationassembly from entering the waveguide, which may be under vacuum. Asshown in FIG. 2, the RF window may be disposed proximate the waveguideand/or attached directly to the coupler. The RF window may includecooling to prevent overheating. The position of the RF window is alsoselected to place it at a minimum of the reflected standing wave fieldsuch that the heating is minimized.

Coupler

A coupler (e.g., coupler cell) may be included as part of the waveguide.The coupler may receive microwaves from the microwave generationassembly and propagate the waves into and/or through the waveguide.

Mechanical Arm

A mechanical (e.g., robotic) arm may be included in a radiotherapysystem. The mechanical arm may provide support and/or movement for thelinac head. Using the mechanical arm, the linac head may be able toreach large ranges of angles of radiation therapy delivery.

Electron Gun

The electron generator 105 (e.g., electron gun, electron emitter) mayproduce electrons along a range of angles. In some embodiments, theelectron generator 105 produces a collimated electron beam. The electronbeam may be configured to produce a range of kinetic energies. Electrongenerators may be classified by the type of electric field generation(DC or RF), by emission mechanism (thermionic, photocathode, coldemission, plasmas source), by focusing (pure electrostatic or withmagnetic fields), or by the number of electrodes.

FIG. 3 illustrates an energy level range of some electrons produced byelectron generator 105 passing through waveguide apertures 302. As shownin FIG. 3, the beam 300 begins at 304 as a relatively low-energy beam,accelerating to a higher energy level at 306 traveling through thewaveguide. FIG. 4 illustrates an example coupling of two parts of oneembodiments of a linear accelerator head. A linear accelerator head mayuse an electron generator 105, such as an L-3 M592 or other model. Acathode may be separated from an anode by a distance of between about 4mm and 22 mm (e.g., about 10 mm). The electron gun may provide, forexample, a 15 keV DC beam that can be focused into an aperture of theaccelerating section (e.g., waveguide) of the linac head, for example,at output aperture 107. The aperture may have a diameter of betweenabout 1 mm and 15 mm (e.g., about 2 mm). The emitted current is adjustedby the proper choice of grid voltage in the range of about −65 V (0 A)to 70V (1.4 A). Other ranges are also possible. The performance of someembodiments of the electron gun and the beam quality can be verifiedwith beam dynamics simulations, using codes such as EGUN, CST Particle,and PARMELA.

Electron Gun Vacuum Flange

An electron gun vacuum flange 117 can be configured improve vacuumconductance in the beginning of the waveguide 110 (e.g., standingwaveguide or SWG). An electron gun vacuum flange 117 (e.g., electron gunvacuum flange) may be oriented so that it can generate fluid flow froman interior of the electron gun chamber and/or the waveguide 110. Insome designs, the waveguide 110 is comprised of cells that include anaperture having a diameter of less than about 1 cm. Due to fluid dynamicrestrictions on fluence for apertures smaller than about 1 cm, theelectron gun vacuum flange can provide fluid communication between avacuum pump and the interior of the electron gun chamber and/or thewaveguide 110. In some embodiments, the aperture is about 4 mm indiameter. The diameter may be between about 1 mm and 7 mm.

The electron gun vacuum flange 117 may be disposed between electrongenerator and the waveguide. Accordingly, the electron gun vacuum flange117 includes a waveguide coupling section 152, a waist 154, and a vacuumpump coupling section 156. An aperture 158 is in fluid communicationwith the interior of the waveguide 110 and the vacuum pump couplingsection 156, such that a vacuum pump coupled to the vacuum pump couplingsection 156 (e.g., directly or by an intermediate conduit) caneffectively remove gases or particles from the interior of the waveguide110 at the electron gun end of the waveguide 110. In some embodiments,the electron gun vacuum flange 117 includes an aperture 111 that iscoaxial with a beam axis along a beam of electrons. In some embodiments,the electron gun vacuum flange 117 defines one or more channelsconfigured to pass fluid therethrough.

Waveguide

The waveguide 110 (e.g., standing waveguide or SWG) may be made ofcopper and may have a length of between about 20 cm and 75 cm. In somedesigns, the waveguide is about 30 cm long. The waveguide may be able toaccelerate electrons having an initial energy of about 10 keV to about 6MeV at the output of the waveguide.

FIG. 6 illustrates an example standing waveguide 110 in accordance withthe linac head embodiments disclosed herein. In some embodiments, thewaveguide 110 includes one or more coupling cells 172 (e.g., side cells,sub-cells, etc.). The coupling cells 172 may be disposed on one oranother side of the waveguide 110 (e.g., opposite sides) and may beoffset from corresponding coupling cells on an opposite side of thewaveguide 110 by an offset distance. The offset distance may be tuned toallow a standing wave to oscillate within the waveguide 110. Thewaveguide can be configured to achieve one or more modes (e.g., resonantfrequencies), such as, for example, π or π/2. Bi-directional tuners 176may be included on the side cells. One or more cooling blocks 160 may bedisposed on the top and/or the bottom of the standing waveguide 110which prevents the waveguide from overheating and keeps the frequency ofthe accelerating mode within the range of tunability of the RF powersource. Since the waveguide 110 may be smaller than traditionalwaveguides used in medical radiotherapy, a relatively large surface areaof the one or more cooling blocks 160 can be in thermal communicationwith the waveguide 110. The cooling block(s) 160 may provide temperaturecontrol for the waveguide 110. In some embodiments, the cooling blocks160 may include additional holes, grooves and/or slots, for example, toprovide access to a user to one or more elements of the waveguide 110(e.g., tuners, shorting openings). Such grooves and/or slots may provideaccess to certain portions of one or more cells of the waveguide 110,for example, to tune them (e.g., through the tuning pins). In someembodiments, tuning may be performed after the one or more coolingblocks 160 have been attached. Coolant conduits 162 in communicationwith interior volumes of the cooling blocks 160 allow a common quantityof coolant to flow through the cooling blocks 160, although the coolingblocks 160 may be separately cooled in some embodiments.

Without being limited by theory, it is believed that reducing the lengthof the waveguide can require smaller apertures within the acceleratingcells of the waveguide. Smaller apertures can further reduce fluence(e.g., for a vacuum) through the waveguide. Therefore, for waveguides ofcertain dimensions, if one large pump is used, there may be insufficientvacuum conductance in parts of the waveguide (e.g., near the electrongenerator and/or near the converter). Accordingly, two or more vacuumports/flanges may be used through which vacuum generation within thewaveguide may be achieved. For example, an electron gun vacuum flange117 may be included near the electron generator (e.g., between theelectron generator and the waveguide), a converter vacuum flange 119 maybe included near the converter (e.g., between the waveguide and theconverter) (see, e.g., FIGS. 21-22), and/or a coupler 145 may beincluded to allow vacuum generation through one or more of the cells ofthe waveguide 110 (e.g., through a coupling cell).

Operation of the system (e.g., at the cathode of the electron beam) mayproduce heat that can raise the temperature of the material of the cells(e.g., copper). The heat may also allow gases to be liberated frommaterials in the system. This may increase the vacuum pressure (e.g.,reduce vacuum). This heat may be produced, for example, by the electrongun and/or by the converter. The electron gun may include a cathode thatmay be heated by a filament to raise a temperature of the cathode toraise the energy electrons within the cathode above the work function ofthe cathode, in order to extract them for propagation through thewaveguide. At the converter, electrons may be incident on a disc (e.g.,a foil). The disc may comprise one or more materials, such as, forexample, tungsten, lead, aluminum, copper, an alloy comprising metalelements, etc. When electrons are incident upon the disc, heat isproduced. It may be advantageous to draw the heat produced away from theconverter, which may be done by including cooling channels within oraround the converter.

A relatively short waveguide 110 may promote higher electric fieldstherein. The higher electric fields may require high or ultrahigh vacuumlevels, with pressure of 10⁻⁷ Torr or lower, in order to avoid breakdownof the electric field on the surface of the waveguide. Combined with asmaller aperture within the waveguide 110, it can be advantageous toevacuate the waveguide 110 and/or other components of the system.Accordingly, the vacuum flanges 117, 119 may be installed at one or moreends of the waveguide. This may provide a desired vacuum conductance atcertain portions of the system, (e.g., one or more ends of the waveguide110).

Tuning features may be included in the system. In some embodiments,tuning features are included in one or more cells of the waveguide, suchas the accelerating cells (e.g., bunching cell(s), coupler cell(s),standard cell(s)) and/or side cells (e.g., coupling cells). The tuningfeatures may provide, for example, the side cells with a correctfrequency of microwave. In some designs, the accelerating cells may bemore precisely tuned. The tuning features may provide structuresconfigured to allow a user to tune one or more of the waveguide cells.

A tuning process may include tuning one or more of the side couplingcells 172. The side cells may be tuned separately. This may beaccomplished by using an antenna assembly to indicate to what extent thecell should be tuned. Accordingly, the side cell may be tuned to thecorrect frequency. In some implementations, an object such as a shortingpin may be inserted into one or more of the side coupling cells. Ashorting pin may be inserted through openings 174 in the side of the oneor more side coupling cells 172. Insertion of the shorting pin may allowthe side coupling cells 172 to be electrically shorted. In may beadvantageous to electrically short one or more of the side couplingcells 172 during tuning of one or more of the accelerating cells. Forexample, shorting one or more of the side coupling cells 172 may allowfor more accurate measurements to be received from only the acceleratingcells. Tuning one or more of the accelerating cells may be performed bylooking at the “field balance,” a measurement of the longitudinalelectric field amplitude along the length of the waveguide 110. Anantenna assembly and/or a “bead pull” system may be used for tuning theone or more accelerating cells. This may be done while one or more sidecoupling cells 172 are shorted.

FIG. 7 is a cutaway view illustrating an example waveguide 110. Thewaveguide may contain fewer than 24 accelerating cells 170. In somedesigns, the waveguide contains 19 cells. The cells may be machined outof metal (e.g., copper, a copper alloy). The length of the waveguide 110may be less than about 70 cm. In some embodiments, the waveguide has alength of about 30 cm. The waveguide 110 may include shorter bunchingcells 178 in addition to standard accelerating cells 170.

The accelerating cells 170 may also be configured to be bi-directionallytuned using, for example, tuning studs 180. The tuning studs aredepicted and described in greater detail below with reference to FIGS.6, 7, 12B, 12C and 14. Two or more tuning studs 180 may be included on acell. As described above, the cooling block 160 includes openings 164 topermit access to the tuning studs 180. Including a plurality of tuningstuds 180 may allow for a greater range of frequency to be tuned, orfiner tuning and/or accuracy in tuning. In some embodiments, the tuningstuds 180 allow a user to attach a tuning attachment that is configuredto allow the user to deform the cell (e.g., accelerating cell 170, sidecell). The deformation of the cell may be accomplished using, forexample, a slide hammer or other implement. The tuning studs 180 (FIG.7), 176 (FIG. 6) may be threaded to facilitate coupling of the slidehammer or other tuning implement. In some embodiments, the one or morecells are configured to be tuned once before use of the system. Otherimplementations are possible.

The accelerating cell tuning studs 180 may be brazed along a top and/orbottom of the waveguide 110. For example, the tuning studs 180 may bebrazed into pockets 179 located along the waveguide 110. This may bedone at an equator of one or more of the cells. In some embodiments, thepockets 179 may be created after assembly of the waveguide 110 (e.g.,after the accelerating cells 170 are coupled together), and prior tobrazing the accelerating cell tuning studs 180. For example, amanufacturing process may include assembling the accelerating cells 170and the cooling block 160 (which may already contain openings 164), suchas by brazing, followed by machining the pockets 179, then brazing thetuning studs 180 within the machined pockets 179. Other manufacturingprocesses are possible. One or more of the side cell studs 176 (FIG. 6)may be brazed along the axes of the side cells. In some embodiments, thepumping flange and converter flange may be welded (e.g., tungsten inertgas (TIG) welded, electron beam welded, etc.) to the waveguide 110 afterit is tuned. As described above, one or more side cells may include anopening for insertion of shorting pins for tuning purposes. Aftertuning, the opening in the one or more cells may be sealed (e.g.,hermetically) with a plug. The plug may be welded to cover the opening.

In some embodiments, one or more cells of the waveguide 110 are brazedto one or more vacuum flanges (e.g., electron gun flange, converterflange). The vacuum flange may include a conflat flange. A side coupledstructure comprising one or more side cells may be used to achieve π/2standing wave mode. FIG. 8 shows an example geometry of a waveguide. Themicrowave generator may produce a power greater than 1 MW. In someimplementations, the microwave generator can produce a power of about 2MW. The microwave generator may produce microwaves at a frequency ofbetween about 7.0 and 11.2 GHz. In some designs, the microwave generatorgenerates microwaves at about 9.3 GHz.

A structural analysis may be performed on various parts of the system toestimate the adverse effects of heating of various components of awaveguide. FIG. 9 shows an example of a structural analysis of awaveguide, illustrating thermal expansion of various portions of thewaveguide due to heating.

Referring now to FIGS. 10 and 11, in order to accelerate the electronsfrom the energy provided by the electron gun, e.g., about 15 keV, to anenergy at which the electrons are traveling at highly relativisticvelocities, e.g., about 1 MeV, one or more bunching cells 178 may beprovided. FIG. 10 schematically illustrates three bunching cellsfollowed by a standard accelerating cell 170. Apertures 181 permit theflow of electrons between the cells 170, 178. The bunching cells 178 mayhave a shorter length (e.g., defined along a beam axis of the system)compared to standard accelerating cells 170. The bunching section mayproduce a transmission efficiency of about 40% for an accelerationgradient of between about 15 MV/m and 30 MV/m. The electric fielddistribution within the depicted portion of the waveguide is indicatedby lines 1000.

FIG. 11 shows the effects of tuning. The shape of a cell (e.g.,accelerating cell 170) may be altered to increase shunt impedance. Asdescribed above, the cells may be tuned using tuning studs disposed atthe exterior of the waveguide. Manipulation of the tuning studs can movethe cell wall 171 inward or outward. For example, the cell 170 of FIG.11 is altered by moving the cell walls 171 outward. Cell 170′ has cellwalls 171′ extending further from the central axis. An example of theresulting alteration of electric field distribution is shown in FIG. 11.

FIGS. 12A-12C illustrate a waveguide with enlarged portions illustratingexample tuning structures in FIGS. 12B and 12C as described above. FIGS.13A and 13B illustrates cell geometries with accelerating cells 170,side coupling cells 172, along with some milled portions duringmanufacture. FIG. 14 is a cutaway view further illustrating exampletuning structures (in the enlarged cutaway section of bunching cells178, coupling cells 172, and accelerating cells 170), as describedabove.

Cooling System

FIG. 15A illustrates an example cooling system that includes coolingblocks 160 (e.g., a cell plate) with a channel allowing fluid to flowtherethrough. Conduits 162 permit a common quantity of coolant to flowbetween the cooling blocks 160. In some embodiments, conduits 162 mayfurther be configured to carry coolant to and from other coolingcomponents, for example, coolant paths for cooling additional componentssuch as the electron generator, converter, or other components of thelinac head (not shown in FIG. 15A) that may generate heat duringoperation and/or that may have a desired operating temperature range.FIG. 15B shows relative heat distribution in one embodiment of awaveguide with a cooling system consistent with the cooling systemdepicted in FIG. 15B. As shown by FIGS. 15A and 15B, the waveguide maybe surrounded on opposite sides by a cooling plate. A relatively highertemperature may be produced in and/or near the bunching cells at theelectron generator end of the waveguide 110 due to, for example, thehigher power density present at this location.

FIGS. 16A and 16B illustrate a further embodiment of an examplewaveguide with associated heat signatures. Waveguides that propagatemicrowaves in the X band frequency range (e.g., between about 7.0 GHzand 11.2 GHz) may promote smaller apertures within the waveguide than,for example, microwaves in the S band frequency range. Smaller apertureswithin the waveguide can reduce fluence from an interior of thewaveguide to a vacuum pump. As described above, use of vacuum flanges atone or both ends of the waveguide may promote a lower pressure withinthe waveguide by, for example, increasing fluence of gas particles outof the waveguide. This may also help improve vacuum pressure at one orboth ends of the waveguide.

Converter

As described above, and with further reference to FIGS. 17 and 18, aconverter for producing photons may be provided at an end of thewaveguide opposite the electron generator. FIG. 17 illustrates anexample vacuum flange which may be included as the converter vacuumflange 119 configured to facilitate vacuum pumping at the converter endof the waveguide. In some embodiments, the flange depicted in FIG. 17may also be included as the electron gun vacuum flange 117 as describedabove. FIG. 18 is a cutaway perspective view illustrating the converterend of the waveguide, including the converter vacuum flange 119 andconverter 120 coupled thereto. The converter vacuum flange 119 generallyhas a similar structure and function to the electron gun vacuum flange117 described elsewhere herein. For example, the converter vacuum flange119 includes a waveguide coupling section 182, a waist 184, and a vacuumpump coupling section 186. An aperture 188 extends through the waist 184and the vacuum pump coupling section 186 to provide fluid communicationbetween the interior of the waveguide and a vacuum pump. Apertures 189within the waveguide coupling section provide a fluid flow path betweenthe waveguide interior and the aperture 188.

The converter is disposed at the end of the accelerating waveguideopposite the electron generator. In some embodiments, the converter 120may be included within a collimator, or may be a standalone structureupstream from a collimator. Collimators are described in greater detailbelow with reference to FIG. 19. For example, the converter 120 may bebetween the collimator and a waveguide exit. The converter 120 mayinclude a disc 122 (e.g., a metallic foil) disposed within an interiorspace 124 of the converter 120. In some embodiments, the converter disc122 has a thickness of between about 1 mm and 8 mm. The converter mayhave two or more layers. A first layer may have a thickness of betweenabout 0.5 mm and 4 mm. A second layer may have a thickness of betweenabout 0.5 mm and 4 mm. The first layer may be comprised of a high Zmaterial, such as tungsten, lead, or other high Z metal. The secondlayer may be comprised of a low or moderate Z material, such as aluminumcopper, or the like. However, other materials may equally be used. Thedisc may receive incident electrons and convert them to photons (e.g.,x-rays).

Collimator

FIG. 19 depicts an example collimator 125 for the photon beam producedat the converter as described above. A first collimator 125 may includea cylinder head 127 with a diameter of between about 80 mm and 130 mm.In some designs, the diameter of the cylinder head 127 is about 110 mm.In some embodiments, the cylinder head 127 is configured to reduceleakage of photons from the system, for example, to less than about 10mGy/min at 1 m from the isocenter. A removable metal (e.g., tungsten)“plug” 129 may be provided which specifies the final field shape ataperture 128 and can be exchanged for other field sizes. In someembodiments, the plug 129 produces a rectangular field size having alength and/or width of up to 40 cm at 1 m from the converter.

Manufacturing

With continued reference to the drawings, various manufacturingconsiderations will now be described. In some implementations,individual cell components may be machined and cleaned. The cellcomponents may then be joined with a brazing material and brazed (e.g.,heated) to join the components together. The brazing material maycomprise a material with a melting temperature lower than that of thematerial used in the cell components. The cell components may be made,for example, of metal such as copper. The brazing material may comprisean alloy, such as a copper alloy. In some embodiments, the brazingmaterial comprises gold. Silver may also be used.

A first group of system components (e.g., cell components) may be brazedusing a first brazing material. In some embodiments, the first brazingmaterial includes an alloy. The first brazing material may be comprisedof between about 50% and 90% of a first metal by weight. The firstbrazing material may be comprised of between about 10% and 50% of asecond metal. The alloy metal has a lower melting then the cellmaterial. The first group of system components may be brazed at a firstbrazing temperature. The first brazing temperature may be between about900° C. and 1050° C.

A second group of system components may be brazed using a second brazingmaterial. The second brazing material may be comprised of a lowerpercentage of the first metal by weight than the first brazing material.In some embodiments, the second brazing material includes a higherpercentage of the second metal by weight than the first brazingmaterial. The second brazing material may be comprised of between about35% to 75% by weight of the first metal. The second brazing material maybe comprised of between about 65% and 25% by weight of the second metal.Metals other than the first metal in second metal may be used for thesecond brazing material. The second group of system components may bebrazed at a second brazing temperature. The second brazing temperaturemay be lower than the first brazing temperature. For example, the secondbrazing temperature may be between about 750° C. and 1050° C.

A third group of system components (e.g., water cooling plate(s)) may bebrazed using a third brazing material. The third brazing material may becomprised of a lower percentage of the first metal by weight than thefirst brazing material and/or than the second brazing material. In someembodiments, the third brazing material includes a higher percentage ofthe second metal by weight than the first brazing material and/or thanthe second brazing material. The third brazing material may be comprisedof between about 10% to 50% by weight of the first metal. The secondbrazing material may be comprised of between about 50% and 10% by weightof the second metal. Metals other than the first metal in second metalmay be used for the third brazing material. The third group of systemcomponents may include one or more water cooling plates. The third groupof system components may be brazed at a third brazing temperature. Thethird brazing temperature may be lower than the first brazingtemperature and/or than the second brazing temperature. For example, thethird brazing temperature may be between about 700° C. and 1000° C.

Treatment Systems

These linear accelerator systems depicted herein and described above maywork in conjunction to provide control signals, read back, power, andcontrol interface to produce the required radiation parameters. FIG. 20illustrates an example treatment system 400. The treatment system 400generally includes a treatment room 410, a control room 420, and anequipment room 430. The treatment room may include a linac head 416, forexample, including the various linac head components described above.The treatment room may further include an electron gun driver 412 andmodulator tank unit 414. The control room 420 may include a controlconsole 422. In some embodiments, a pulse transformer tank unit 436 isplaced in the electronics rack 435 with the PLC/controls 438 orelsewhere outside the linac head 416, rather than in the linac head 416.Because the tank unit 436 may be large and/or heavy (e.g., 80 kg), itmay be advantageous not to place the pulse transformer tank in the linachead 416. The equipment room 430 may also include a temperature controlunit 432 and a compressed air supply 434. The temperature control unit432 may be in fluid communication with the electronics rack 435, linachead 416, and/or modulator tank unit 414 by one or more coolant lines,such that the temperature control unit 432 can control the temperatureof the coolant traveling to those components. The linac head 416 may beconnected by control lines to the electron gun driver 412, and modulatortank unit 414. The electronics rack 435 may additionally be connected byone or more control lines to the control console 422 and/or themodulator tank unit 414.

Example Embodiments

1. A linear accelerator head for use in a medical radiation therapysystem, the linear accelerator head comprising:

-   -   a housing;    -   an electron generator configured to emit electrons along a beam        path;    -   a microwave generation assembly comprising:        -   a microwave generator configured to emit microwaves in a            first direction along a primary wave path; and        -   an isolator configured to prevent microwaves from            propagating in a second direction opposite the first            direction along the primary wave path;    -   a waveguide configured to contain a standing or travelling        microwave, the waveguide comprising:        -   a plurality of cells disposed adjacent one another, wherein            each of the plurality of cells defines an aperture            configured to receive electrons therethrough, the aperture            of the plurality of cells having a diameter and defining a            beam axis of the waveguide along the beam path, wherein each            of the plurality of cells has a first length defined along            the beam axis;        -   one or more bunching cells comprising an aperture configured            to accelerate electrons therethrough, wherein each of the            one or more bunching cells has a second length along the            axis smaller than the first length of the plurality of            cells; and        -   coupler cell configured to couple microwaves from the            isolator into the waveguide;        -   wherein the waveguide is configured to accelerate electrons            to between about 3 MeV and 9 MeV;    -   a cooling system in thermal communication with the waveguide;    -   a converter disposed within the electron beam path and        configured to receive incident electrons, wherein the converter        is configured to convert incident electrons into photons; and    -   a first collimator configured to define a beam shape, wherein        the first collimator comprises an input aperture and an output        aperture;    -   wherein the linear accelerator head has a length of between        about 50 cm and 120 cm, a width of between about 40 cm and 90        cm, and a depth of between about 20 cm and 75 cm.

2. The linear accelerator head of embodiment 1, wherein each of theplurality of cells comprises a cell wall with an attached tunerconfigured to deform the cell wall.

3. The linear accelerator head of embodiment 2, wherein the tuner isconfigured to allow a user to deform the cell wall by translating aportion of the cell wall radially toward the beam axis.

4. The linear accelerator head of any of embodiments 2-3, wherein thetuner is configured to allow the user to deform the cell wall bytranslating a portion of the cell wall radially away from the beam axis.

5. The linear accelerator head of any of embodiments 1-4, wherein thewaveguide further comprises a plurality of side cells.

6. The linear accelerator head of embodiment 5, wherein each of theplurality of side cells comprises a tuner configured to allow a user todeform a side cell wall of the side cell by translating a portion of theside cell wall axially parallel to the beam axis.

7. The linear accelerator head of any of embodiments 5-6, wherein eachof the plurality of side cells comprises an opening defining a shortingaxis radial from the beam axis.

8. The linear accelerator head of any of embodiments 1-7, comprising anelectron gun vacuum flange disposed between the electron generator andthe waveguide, the electron gun vacuum flange comprising an electron gunvacuum flange aperture coaxial with the beam axis, wherein the electrongun vacuum flange is configured to provide fluid communication between avacuum pump and an interior of the waveguide.

9. The linear accelerator head of any of embodiments 1-8, comprising aconverter vacuum flange disposed between the waveguide and theconverter, the converter flange comprising a converter vacuum flangeaperture coaxial with the beam axis, wherein the converter vacuum flangeis configured to provide fluid communication between a vacuum pump andan interior of the waveguide.

10. The linear accelerator head of any of embodiments 1-9, wherein themicrowave generator comprises a magnetron.

11. The linear accelerator head of any of embodiments 1-10, wherein themicrowave generator is configured to emit microwaves at a frequency in arange of between about 7.0 GHz and 11.2 GHz.

12. The linear accelerator head of any of embodiments 1-17, wherein themicrowave generator is configured to emit waves at a power greater than1 MW.

13. The linear accelerator head of any of embodiments 1-11 configured todeliver between about 300 cGy/min and 1,800 cGy/min.

14. The linear accelerator head of any of embodiments 1-13, wherein ameans for attaching the electron generator to the waveguide comprises aremovably coupled flange.

15. The linear accelerator head of any of embodiments 1-14 wherein thediameter of the aperture of each of the plurality of cells is betweenabout 0.2 cm and 1.0 cm.

16. The linear accelerator head of any of embodiments 1-15, wherein thecoupler cell is configured to provide fluid communication between avacuum pump and an interior of the waveguide.

17. The linear accelerator head of any of embodiments 1-16, wherein theplurality of cells comprises fewer than 24 cells.

18. The linear accelerator head of any of embodiments 1-7, wherein thewaveguide comprises a first exterior surface parallel to the beam axis.

19. The linear accelerator head of embodiment 18, wherein the waveguidecomprises a second exterior surface, wherein the first exterior surfaceis parallel to the second exterior surface.

20. The linear accelerator head of any of embodiments 18-19, wherein thecooling system comprises a block defining a surface coplanar with thefirst exterior surface of the waveguide, the block comprising a channelconfigured to guide fluid therethrough.

21. The linear accelerator head of any of embodiments 18-20, wherein thecooling system comprises a block comprising a channel configured toguide fluid therethrough, wherein the block comprises an openingconfigured to allow a user access to at least one of the tuners of theplurality of cells.

22. The linear accelerator head of any of embodiments 1-21, wherein theconverter comprises a disc comprising a first layer comprising a firstmaterial and a second layer comprising a second material.

23. The linear accelerator head of embodiment 22, wherein the firstmaterial comprises a chemical element having an atomic number greaterthan about 57.

24. The linear accelerator head of any of embodiments 22-23, wherein thesecond material comprises a chemical element having an atomic numberlower than about 57.

25. The linear accelerator head of any of embodiments 22-24, wherein thefirst material comprises tungsten.

26. The linear accelerator head of any of embodiments 22-25, wherein thesecond material comprises aluminum.

27. The linear accelerator head of any of embodiments 22-25, wherein thesecond material comprises copper.

28. The linear accelerator head of any of embodiments 22-26, wherein thedisc has a thickness of between about 1 mm and 8 mm.

29. The linear accelerator head of any of embodiments 22-28, wherein thefirst layer has a thickness of between about 0.5 mm and 4 mm.

30. The linear accelerator head of any of embodiments 22-29, wherein thesecond layer has a thickness of between about 0.5 mm and 4 mm.

31. The linear accelerator head of any of embodiments 1-30, wherein adiameter of a circle inscribed by the input aperture of the firstcollimator is greater than the diameter of the aperture of each of theplurality of cells.

32. The linear accelerator head of any one of embodiments 1-31, whereina diameter of a circle inscribed by the output aperture of the firstcollimator is greater than the diameter of a circle inscribed by theinput aperture.

33. The linear accelerator head of any of embodiments 1-32, furthercomprising a second collimator, wherein the second collimator comprisesa plurality of collimating leaves.

34. The linear accelerator head of embodiment 33, wherein each of theplurality of collimating leaves is configured to translate along acollimating plane orthogonal to an axis defined by a circle inscribed bythe output aperture.

35. The linear accelerator head of any of embodiments 33-34, whereineach of the plurality of collimating leaves comprises tungsten.

36. The linear accelerator head of any of embodiments 33-35, whereineach of the plurality of collimating leaves comprises lead.

37. A method of manufacturing a linear accelerator head for use in amedical radiation therapy system, the method comprising:

-   -   assembling a waveguide comprising a plurality of cells, wherein        each of the plurality of cells defines an aperture configured to        receive electrons therethrough, the aperture of the plurality of        cells having a diameter and defining a beam axis along a beam        path, the step of assembling a plurality of cells comprising:        -   for each of a first plurality of cells, providing a first            brazing alloy between two adjacent cell members; and        -   heating each of a first plurality of cells to a first            brazing temperature; providing a housing;    -   providing an electron generator configured to emit electrons        along the beam path;    -   providing a microwave generation assembly comprising:        -   a microwave generator configured to emit microwaves in a            first direction along a primary wave path; and        -   an isolator configured to prevent microwaves from            propagating in a second direction opposite the first            direction along the primary wave path;    -   providing a cooling system in thermal communication with the        waveguide;    -   providing a converter disposed within the electron beam path and        configured to receive incident electrons, wherein the converter        is configured to convert incident electrons into photons; and    -   providing a first collimator configured to define a beam shape,        wherein the first collimator comprises an input aperture and an        output aperture.

38. The method of manufacturing a linear accelerator head of embodiment37, wherein the step of assembling the plurality of cells comprises thestep of for each of the first plurality of cells, machining a cavityinto a portion of each of the two adjacent cell members.

39. The method of manufacturing a linear accelerator head of any ofembodiments 37-38, wherein the first brazing alloy comprises betweenabout 50% and 90% copper by weight.

40. The method of manufacturing a linear accelerator head of any ofembodiments 37-39, wherein the first brazing alloy comprises betweenabout 10% and 50% gold by weight.

41. The method of manufacturing a linear accelerator head of any ofembodiments 37-40, wherein the first brazing temperature is betweenabout 900° C. and 1100° C.

42. The method of manufacturing a linear accelerator head any ofembodiments 37-41, wherein the step of assembling a plurality of cellscomprises:

-   -   for each of a second plurality of cells, providing a second        brazing alloy between two adjacent cell members; and    -   heating each of a second plurality of cells to a second brazing        temperature.

43. The method of manufacturing a linear accelerator head of embodiment42, wherein the step of assembling the plurality of cells comprises thestep of for each of the first plurality of cells, machining a cavityinto a portion of each of the two adjacent cell members.

44. The method of manufacturing a linear accelerator head of any ofembodiments 42-43, wherein the second brazing alloy comprises betweenabout 35% and 75% copper by weight.

45. The method of manufacturing a linear accelerator head of any ofembodiments 42-44, wherein the second brazing alloy comprises betweenabout 25% and 65% gold by weight.

46. The method of manufacturing a linear accelerator head of any ofembodiments 44-45, wherein the second brazing alloy comprises a lowerpercentage by weight of copper than the first brazing alloy.

47. The method of manufacturing a linear accelerator head of any ofembodiments 37-46, wherein the step of providing a cooling system inthermal communication with the waveguide comprises:

-   -   providing a third brazing alloy between the waveguide and a        cooling plate, the cooling plate comprising a channel configured        to guide fluid therethrough; and    -   heating the waveguide and cooling plate to a third brazing        temperature.

48. The method of manufacturing a linear accelerator head of embodiment47, wherein the third brazing temperature is between about 700° C. and1000° C.

49. The method of manufacturing a linear accelerator head of any ofembodiments 47-48, wherein the third brazing alloy comprises betweenabout 10% and 50% copper by weight.

50. The method of manufacturing a linear accelerator head of any ofembodiments 47-49, wherein the third brazing alloy comprises betweenabout 50% and 10% gold by weight.

51. The method of manufacturing a linear accelerator head of any ofembodiments 49-50, wherein the third brazing alloy comprises a lowerpercentage by weight of copper than the second brazing alloy.

52. The method of manufacturing a linear accelerator head of any ofembodiments 37-51, further comprising the step of dipping the firstplurality of cells into a solvent configured to dissolve oil.

53. The method of manufacturing a linear accelerator head of any ofembodiments 3749-52, further comprising the step of propagating soundwaves at ultrasound frequency at the first plurality of cells.

54. The method of manufacturing a linear accelerator head of any ofembodiments 3749-50, further comprising the step of dipping the firstplurality of cells into an etching solution configured to remove atleast a layer of copper.

55. The method of manufacturing a linear accelerator head of embodiment54, wherein dipping the first plurality of cells into an etchingsolution comprises dipping the first plurality of cells into the etchingsolution for a total time of at least fifteen seconds.

56. The method of manufacturing a linear accelerator head of any ofembodiments 54-55, wherein the etching solution comprises phosphoricacid.

57. A system for delivering radiotherapy using a linear accelerator, thesystem comprising:

-   -   a linear accelerator head of any of embodiments 1-36;    -   a mechanical arm comprising a hinge defining a axis of rotation,        the mechanical arm configured to rotate about the axis of        rotation;    -   wherein the linear accelerator head is mechanically coupled to        the mechanical arm using a coupling device.

58. The system for delivering radiotherapy using a linear accelerator ofembodiment 57, wherein the beam axis of the waveguide is perpendicularto the axis of rotation.

59. The system for delivering radiotherapy using a linear accelerator ofany of embodiments 57-58, wherein one or more of a proximate end of themagnetic generator and a proximate end of the collimator is not disposedwithin about 15 cm from the axis of rotation.

60. The system for delivering radiotherapy using a linear accelerator ofany of embodiments 57-59, wherein the coupling device is a hinge.

61. The system for delivering radiotherapy using a linear accelerator ofany of embodiments 57-60, wherein an orientation between the linearaccelerator head and the mechanical arm comprises a lower moment ofinertia than an orientation wherein at least one of a proximate end ofthe magnetic generator and a proximate end of the collimator is withinabout 6 cm from the axis of rotation.

62. A compact linear accelerator head comprising:

-   -   an electron generator configured to emit electrons along a beam        path;    -   a microwave generation assembly configured to emit microwaves in        a first direction along a primary wave path;    -   a waveguide configured to contain a standing or travelling        microwave and accelerate electrons to between about 3 MeV and        about 9 MeV, the waveguide comprising:        -   a plurality of adjacent cells, each of the plurality of            adjacent cells defining an aperture having a diameter and            configured to receive electrons therethrough, the apertures            of the adjacent cells defining a beam axis of the waveguide            along the beam path, wherein each of the adjacent cells has            a first length defined along the beam axis;        -   one or more bunching cells comprising an aperture configured            to accelerate electrons therethrough, wherein each of the            one or more bunching cells has a length along the beam axis            smaller than the first length; and        -   a coupler cell configured to couple microwaves from the            microwave generation assembly into the waveguide;    -   a converter disposed within the electron beam path, the        converter configured to receive incident electrons and convert        the incident electrons into photons, the converter comprising a        disc comprising:        -   a first layer comprising a first material; and        -   a second layer comprising a second material; and    -   a first collimator configured to define a beam shape, wherein        the first collimator comprises an input aperture and an output        aperture.

63. A method of manufacturing a linear accelerator head, the methodcomprising:

-   -   assembling a waveguide comprising a plurality of cells, wherein        each of the plurality of cells defines an aperture configured to        receive electrons therethrough, the aperture of the plurality of        cells having a diameter and defining a beam axis along a beam        path, the step of assembling a plurality of cells comprising:        -   for each of a first plurality of cells, machining a cavity            into a portion of each of the adjacent cell members;        -   for each of the first plurality of cells, providing a first            brazing alloy between the two adjacent cell members; and        -   heating each of the first plurality of cells to a first            brazing temperature;    -   providing an electron generator configured to emit electrons        along the beam path;    -   providing a microwave generation assembly comprising:        -   a microwave generator configured to emit microwaves in a            first direction along a primary wave path; and        -   an isolator configured to prevent microwaves from            propagating in a second direction opposite the first            direction along the primary wave path;    -   providing a cooling system in thermal communication with the        waveguide;    -   providing a converter disposed within the electron beam path and        configured to receive incident electrons, wherein the converter        is configured to convert incident electrons into photons; and    -   providing a first collimator configured to define a beam shape,        wherein the first collimator comprises an input aperture and an        output aperture.

64. The method of embodiment 63, wherein the first brazing alloycomprises between about 50% and about 90% copper by weight.

65. The method of embodiment 63, wherein the first brazing alloycomprises between about 10% and about 50% gold by weight.

66. The method of embodiment 64 or 65, further comprising dipping thefirst plurality of cells into an etching solution configured to removeat least a layer of copper.

67. The method of embodiment 63, further comprising dipping the firstplurality of cells into an etching solution configured to remove atleast a layer of a constituent element of the first brazing alloy.

68. The method of embodiment 66 or 67, wherein the first plurality ofcells are dipped into the etching solution for a total time of at leastthirty seconds.

69. The method of embodiment 66, wherein the etching solution comprisesphosphoric acid.

70. The method of embodiment 63, wherein the first brazing temperatureis between about 900° C. and about 1100° C.

71. The method of embodiment 63, wherein providing the cooling system inthermal communication with the waveguide comprises:

-   -   providing a third brazing alloy between the waveguide and a        cooling plate, the cooling plate comprising a channel configured        to guide fluid therethrough; and    -   heating the waveguide and cooling plate to a third brazing        temperature.

72. The method of embodiment 63, further comprising, subsequent toassembling the plurality of cells:

-   -   machining an opening into an exterior portion of at least one of        the adjacent cell members, the opening extending inward toward        the cavity of the cell member perpendicular to the beam axis;        and    -   brazing a tuning stud in a location at least partially within        the opening, the tuning stud configured to deform at least a        portion of a cell wall of the cavity.

73. A system for delivering radiotherapy using a linear accelerator, thesystem comprising:

-   -   a compact linear accelerator head comprising:        -   an electron generator configured to emit electrons along a            beam path;        -   a microwave generator configured to emit microwaves in a            first direction along a primary wave path;        -   a waveguide configured to contain a standing or travelling            microwave along a beam path and accelerate electrons to            between about 3 MeV and about 9 MeV;        -   a cooling system in thermal communication with the            waveguide;        -   a converter disposed within the electron beam path and            configured to receive incident electrons and convert the            incident electrons into photons;        -   a collimator configured to define a beam shape, the            collimator comprising an input aperture and an output            aperture; and        -   a housing at least partially surrounding the electron            generator, the microwave generator, the waveguide, the            cooling system, the converter, and the collimator, wherein            the housing has a length of between about 50 cm and about            120 cm, a width of between about 40 cm and about 90 cm, and            a depth of between about 20 cm and about 75 cm; and    -   a mechanical arm comprising a hinge defining an axis of        rotation, the mechanical arm configured to rotate about the axis        of rotation;    -   wherein the linear accelerator head is mechanically coupled to        the mechanical arm by a coupling device, and    -   wherein the beam axis of the waveguide is perpendicular to the        axis of rotation of the mechanical arm.

74. The system of claim 6, wherein one or more of a proximate end of themicrowave generator and a proximate end of the collimator is notdisposed within about 15 cm from the axis of rotation.

Other Embodiments

Conditional language, such as, among others, “can,” “could,” “might,” or“may,” unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or steps. Thus, such conditional language is notgenerally intended to imply that features, elements and/or steps are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or without userinput or prompting, whether these features, elements and/or steps areincluded or are to be performed in any particular embodiment.

It should be emphasized that many variations and modifications may bemade to the above-described embodiments, the elements of which are to beunderstood as being among other acceptable examples. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure. The foregoing description details certainembodiments of the invention. It will be appreciated, however, that nomatter how detailed the foregoing appears in text, the invention can bepracticed in many ways. As is also stated above, the use of particularterminology when describing certain features or aspects of the inventionshould not be taken to imply that the terminology is being re-definedherein to be restricted to including any specific characteristics of thefeatures or aspects of the invention with which that terminology isassociated. The scope of the invention should therefore be construed inaccordance with the appended embodiments and/or claims and anyequivalents thereof.

What is claimed is:
 1. A linear accelerator head comprising: an electrongenerator configured to emit electrons along a beam path; a microwavegeneration assembly comprising: a microwave generator configured to emitmicrowaves in a first direction along a primary wave path; and anisolator configured to prevent microwaves from propagating in a seconddirection opposite the first direction along the primary wave path; awaveguide configured to contain a standing or travelling microwave andaccelerate electrons to between about 3 MeV and about 9 MeV, thewaveguide comprising a plurality of cells each defining an apertureconfigured to receive electrons therethrough; and a converter disposedwithin the electron beam path and configured to receive incidentelectrons and convert the incident electrons into photons, the converterincluding a disc comprising: a first layer of a first material; and asecond layer of a second material.
 2. The linear accelerator head ofclaim 1, wherein the first material comprises a chemical element havingan atomic number greater than about
 57. 3. The linear accelerator headof claim 1, wherein the second material comprises a chemical elementhaving an atomic number greater than about
 57. 4. The linear acceleratorhead of claim 1, wherein the first material comprises tungsten.
 5. Thelinear accelerator head of claim 1, wherein the second materialcomprises aluminum.
 6. The linear accelerator head of claim 1, whereinthe second material comprises copper.
 7. The linear accelerator head ofclaim 1, wherein the disc has a thickness of between about 1 mm and 8mm.
 8. The linear accelerator head of claim 1, wherein the first layerhas a thickness of between about 0.5 mm and 4 mm.
 9. The linearaccelerator head of claim 1, wherein the second layer has a thickness ofbetween about 0.5 mm and 4 mm.
 10. A linear accelerator head comprising:an electron generator configured to emit electrons along a beam path; amicrowave generation assembly comprising: a microwave generatorconfigured to emit microwaves in a first direction along a primary wavepath; and an isolator configured to prevent microwaves from propagatingin a second direction opposite the first direction along the primarywave path; a waveguide configured to contain a standing or travellingmicrowave and accelerate electrons to between about 3 MeV and about 9MeV, the waveguide comprising a plurality of cells, each of theplurality of cells defining an aperture having a diameter and configuredto receive electrons therethrough, the apertures of the cells defining abeam axis of the waveguide along the beam path; a cooling system inthermal communication with the waveguide, the cooling system comprisinga block having a channel configured to guide fluid therethrough; and aconverter disposed within the electron beam path, the converterconfigured to receive incident electrons and convert the incidentelectrons into photons.
 11. The linear accelerator head of claim 10,wherein the waveguide comprises a first exterior surface parallel to thebeam axis.
 12. The linear accelerator head of claim 11, wherein thewaveguide comprises a second exterior surface parallel to the firstexterior surface.
 13. The linear accelerator head of claim 12, whereinthe block defines a surface coplanar with the first exterior surface ofthe waveguide.
 14. The linear accelerator head of claim 10, wherein theblock comprises an opening configured to allow a user access to at leastone of the tuners of the plurality of cells.
 15. A linear acceleratorhead comprising: an electron generator configured to emit electronsalong a beam path; a microwave generation assembly comprising: amagnetron configured to emit microwaves in a first direction along aprimary wave path; and an isolator configured to prevent microwaves frompropagating in a second direction opposite the first direction along theprimary wave path; a waveguide configured to contain a standing ortravelling microwave and accelerate electrons to between about 3 MeV andabout 9 MeV, the waveguide comprising a plurality of cells each definingan aperture configured to receive electrons therethrough; and aconverter disposed within the electron beam path, the converterconfigured to receive incident electrons and convert the incidentelectrons into photons.
 16. The linear accelerator head of claim 15,wherein the magnetron is configured to emit microwaves at a frequency ina range of between about 7.0 GHz and 11.2 GHz.
 17. The linearaccelerator head of claim 15, wherein the magnetron is configured toemit microwaves at a power greater than 1 MW.
 18. The linear acceleratorhead of claim 15, configured to deliver between about 300 cGy/min and1800 cGy/min.
 19. The linear accelerator head of claim 15, wherein thewaveguide further comprises a coupler cell configured to couplemicrowaves from the isolator into the waveguide and to provide fluidcommunication between a vacuum pump and an interior of the waveguide.20. The linear accelerator head of claim 15, wherein the plurality ofcells comprises fewer than 24 cells.